So far, numerous techniques have been disclosed about how to form insulating films on the surfaces of electrical steel steers and compositions for providing such insulating films. Long-established techniques make use of insulating film-forming compositions consisting as the main component of chromates and/or phosphates to form inorganic films. A problem with the inorganic films made of such inorganic materials is that although they excel in heat resistance, yet they are inferior in their water resistance, punchability and adhesion to steel sheets during processing.
Recently, according to techniques attempted in many cases to solve such a problem, a film-forming composition containing a mixture of a film forming-inorganic element such as chromate or phosphate etc. (hereinafter sometimes called the inorganic component) with a film forming-organic resin (hereinafter sometimes referred to as the organic component or simply the resin; said resin being usually a synthetic resin which may hereinafter be also called the "polymer") is applied to an electrical steel sheet to form on it a combined inorganic/organic film (hereinafter often sometimes called the semi-organic film). This semi-organic film is to take full advantage of both the properties of the inorganic film that it excels in weldability and heat resistance and the properties of the organic film that it excels in water resistance, insulating performance, punchability and adhesion. A composition for forming the semi-organic film is generally obtained in the form of a mixture (hereinafter sometimes called the inorganic component-containing mixed emulsion or simply referred to as the mixed emulsion) of an aqueous solution (often used with a view to a working environment and fire prevention) of an inorganic component with an aqueous polymer emulsion in which an organic component is dispersed. However, the technique for forming the semi-organic film of the inorganic component-containing mixed emulsion suffers from various disadvantages. The first disadvantage is a short pot life due to the emulsification and dispersion of the resin (polymer) in the mixed emulsion which are unstable. The second disadvantage is that because the mixed emulsion contains an emulsifier or dispersant used to emulsify and disperse the resin (polymer), foams come to be observed on the resulting film by various coating operations such as stirring etc. to give rise to pinholes in crater-forms or bubble-like defects on the film. This makes the formed film poor in corrosion resistance and insulating performance. The third disadvantage is that when electrical steel sheets with films formed on them are punched out followed by lamination and welding, the organic component decomposes to emit gases and give rise to blow-holes in a beaded region, resulting in a serious degradation of weldability.
Action taken against the second disadvantage will now be explained. In order to reduce as much as possible the expansion of the mixed emulsion to a minimum, a defoamer is generally incorporated in the aqueous polymer emulsion that is one preparatory material for the mixed emulsion. However, this defoamer is likely to lose its effect gradually during the storage of the aqueous polymer solution to which the defoamer was added. Especially when it freezes in a cold district during the winter season, it loses its substantial defoaming effect upon thawing. This then causes the mixed emulsion to foam during preparation or when applied to steel plates, giving rise to various difficulties. Another problem with the mixed emulsion is that it is poor in freeze-thaw stability (the dispersion stability of the aqueous polymer emulsion frozen during storage--an index to whether or not it maintains the prior-to-freeze dispersion state after thawing) or it undergoes a drop-with-time of dispersion stability. Such problems have yet to be solved to satisfaction.
In order to eliminate the aforesaid disadvantages relating to pot life, foaming and weldability, the inventors have proposed to use as the organic component a combination of an acryl base resin and/or an acryl/styrene base resin with an acrylonitrile base resin, as set forth in Japanese Patent Laid-Open No. 62-100561.
However, it has now been found that in post-treatments following the formation of insulating films on the surfaces of steel sheets (e.g. annealing which is carried out to rid the steel sheets of deformation caused by punching them into the desired shape and which, by way of example, is performed at 815.degree. C. for 1 hour in a gaseous mixture of 5% H.sub.2 with the balance of N.sub.2), the acrylonitrile base resin decomposes thermally into CO, CH.sub.4 and other gases, which in turn cause an increase in the amount of C in the steel sheets or, to put it another way, give rise to the so-called "carburizing". This carburizing or the increase in the amount of C in the steel sheets deteriorates their magnetic properties severely, thus leading to another problem.
With current tendencies toward making the performance of electrical steel sheet products higher, insulating films for them are increasingly required not only to be enhanced in terms of General characteristics but also to be greatly improved in terms of specific film characteristics, e.g. weldability and insulating performance in particular. If higher insulating performance are to be imparted to such a conventional semi-organic film in meeting such requirements as referred to above, it is necessary to make insulating films thicker. However, imparting a thickness of, say 2-8 g/m.sup.2 or more to an insulating film would result in a flow of a larger amount of the composition for forming it; this implies the liklihood of foaming of that composition. Thus, the film forming-composition is required to be less likely to foam. The increase in the thickness of the film, on the other hand, brings about a considerable drop in its weldability. This is because such a thickness increase gives rise to an increase in the amount of resin to be contained in the insulating film. The resin then decomposes by welding heat to emit a large quantity of gases, which render it difficult to form normal welding beads. As means for solving this problem, there is known a technique according to which coarse resin powders are added to an insulating film to impart a roughness of a minute degree onto the film, whereby the gases produced by welding ape dissipated (which is called as space effect) to improve its weldability. However, this technique is far from satisfactory, because at a degree of roughness defining a space factor of about 99% usually demanded in the art, voids to the quantity of gases by decomposition are so small that any sufficient space effect is not attained. As means for solving this, there is known a technique according to which inorganic coarse powders giving no emission of gases are incorporated in an insulating film. Depending upon the type of the inorganic coarse powders incorporated, however, it gives rise to a serious drop in punchability and, due to their inferior resistance to acids and alkalis, it reacts with, e.g. CrO.sub.3 in a state of composition, resulting in coagulation or a considerable decrease in the pot life of a composition containing them. Furthermore, the incorporation of such powders in a composition brings about an increase in its thickness and hence a decrease in its adhesion. To improve the weldability of an insulating film, on the contrary, by making it by far thinner, e.g. imparting a thickness of, say, 1 g/m.sup.2 or less to it, would reduce its interlaminar resistance (as measured according to JIS C2550)--an index to insulating performance--to a low value of, say, 70-80 .OMEGA..multidot.cm.sup.2 /sheet, and this would cause a drop in its punchability.
Thus, although, taken altogether, the prior art semi-organic films are more or less improved in terms of insulating film properties, yet the compositions for forming them are still not sufficient in terms of foaming, if higher-performance levels currently required for the product are taken into consideration. Nor are the resulting insulating film properties satisfactory. The conventional compositions has an additional disadvantage of carburizing incidental to the decomposition of resins.